need pressure switch help

[WellConfused]

I just drove a 3 foot 1.25" point 17 feet down and have 4 ft water in it. A friend lent me an old shallow well pump that I hooked up without a tank, and it pumps a constant flow of clear water. I just received a new Wayne 1/2 hp pump, and I have a tank (air bladder) about 15 gallons. This well to be used for lawn and garden watering with a hose. I'm confused about the tank/pressure switch arrangement.

First off, the pump will be running continuosly when in use. I plan to install an on/off switch in the electric line. I want the pressure switch to shut down the pump should I temporarily close the hose nozzle. Also, I want the pump to shut down should the well get ever get pumped dry.

I know that if I close the exit line the pump will just cycle and not shut down. For the life of me, I can't understand how adding a tank will shut down the pump as pressure would build up just as if there were no tank. Also, I'm told that the pressure switch that came on the pump will not shut it down should the well go dry, and that I should therefore remove the switch and install a low pressure cut off switch.

Can anyone set me straight on this? Thanks for any help.

[captwally]

For starters, the pressure switch will not shut down your pump should the well draw-down exceed your well depth. That is not what it is designed to do. Hence the name "pressure switch." It will shut the pump off when a certain pressure has been pumped into a tank. If there is no water coming into the pump there is no pressure building up and you run the very serious risk of running your pump dry for an extended period of time resulting in damage to the pump.

The pressure tank will activate the switch if properly adjusted according to the capacity of your pump. This will prevent damage should you close the water flow, ie stop using the water hose. As for shutting the pump down if the water level draw-down exceeds the depth of your well, I have no easy answer for you.

[Pumpman]

There are switches on the market that have a low pressure dropout. Some Sq. D Pumptrol's come with it. The idea is that if a line ruptures (or the well goes dry), the pressure will drop quickly and the switch will sense this and shut the pump down. They do work.
When you run a pump without a pressure tank it will indeed cycle rapidly if you shut off the hose. The pressure builds up quickly, the switch shuts the pump off, but the pressure subsides just as quickly and the switch, sensing the pressure loss, restarts the pump. Bad for motor and switch. The pressure tank acts as a "buffer". The pressure builds up much more slowly, meaning the switch will shut off when cutout is reached. By the same token, the pressure will drop more slowly if water is used and the switch will start the pump when the cutin pressure is reached. Pressure switches are generally in the 20/40, 30/50, or 40/60 ranges.
Ron

[WellConfused]

Thanks for the help. I replaced my pressure switch with a Square D 30/50 switch that has a low pressure cut-off.